Support update builds.

src/config/target.rs

@@ -126,6 +126,13 @@ impl TargetConfig {
     pub(crate) fn get_name(&'_ self) -> Result<&String, CustomError> {
         Ok(&self.name)
     }
+
+    /// Whether this job should update the flake inputs before the build.
+    ///
+    /// This is used for proactively building updated packages so we can quickly update machines without waiting.
+    pub(crate) fn get_update(&self) -> bool {
+        self.update.unwrap_or(false)
+    }
 }
 
 fn hex_string(data: &[u8]) -> String {